Iron chelates of aminopolycarboxylic acids, e.g. ethylenediaminetetraacetic acid, are useful in numerous applications. Their aqueous solutions are employed in photographic processes, as described in U.S. Pat. No. 3,293,036, and for application to iron-deficient soils to provide that element to plants grown therein. Other uses include the removal of H.sub.2 S from gas streams and as a catalyst in aqueous system processes.
In U.S. Pat. No. 3,767,689 a method for preparing aqueous solutions of iron chelates of aminopolycarboxylic acids is described. The method involves heating iron oxide and the aminopolycarboxylic acid in an aqueous medium and subsequently neutralizing with a base. The parameters employed in that process are: heating to a temperature of from 80.degree. C. to 120.degree. C.; a reaction period of five to 20 minutes or up to several hours with more difficulty soluble forms of iron oxide; employing about 10% excess of the aminopolycarboxylic acid over that stoichiometrically required to react with the iron oxide; and employing a ratio of 0.1 to 1 mole NH.sub.4 OH per mole of aminopolycarboxylic acid (preferably 0.8 mole to form the iron-ammonium chelate and then about 1.5 moles additional to bring pH to 7 and insure neutralization of Fe.sup.+++ chelate).
In the above patented method several problems are encountered: (1) excessive foaming and sludge formation is observed during the initial reaction of the aminopolycarboxylic acid with the iron oxide and ammonium hydroxide upon heating; (2) the end product contains about 20% of the iron in the ferrous (Fe.sup.++) form; (3) the attempted oxidation of the ferrous iron to ferric by bubbling air through the reaction mixture during the reaction at elevated temperatures apparently decomposes the aminopolycarboxylic acid, thus reducing the amount of total chelant (the multidentate ligand molecule of the chelating agent) available to form the iron-ammonium chelate.